The exact positioning, time and velocity of an object, such as a projectile, relative to, or passing through, a real surface or an imaginary surface, such as an active optical screen, is important for determining the timing and flight trajectory of the object. Such screens have applications in the study of the dynamics of projectiles and in the protection of stationary or moving targets against projectiles sent toward the targets.
Some of the common methods for determining position, time and velocity are as follows:                i) fast photography of the projectile, using two consequent exposures at a known time delay, and        ii) consumable screens which are torn by the projectile, namely, an electrical or optical conductor screen or screens placed in the trajectory of the projectile.        
The first of the above methods requires a bully and expensive fast camera, whereas the second method is low priced, but requires replacement after every event and cannot perform as a permanent, multi-shot, measurement set up. The need for a re-usable, multi-shot, small and simple system calls for a novel system and method.
Optical or laser screens were proposed in the past. Generally, such systems fall into two categories of systems: a) active systems based on signal transmission towards an object and detection of signals reflected or scattered from the object, and b) passive systems that do not utilize energy transmission towards a target to be detected. A screen for traffic warning, according to their colour, is described in Japanese Patent Application No. 6,119,592. Similar kinds of screens proposed for use by pilots, such as landing strip guides, are described in DE Patent 19930096. U.S. Pat. No. 5,554,262 describes a system using a permanent screen, having a laser transmitter on one side and a laser receiver on the other, for maintaining the right position of paper edge in papermaking machines. The use of optical screens made of optical lines running back and forth between two mirrors is described in U.S. Pat. Nos. 4,097,800 and 6,259,365, where any interruption of the screen results in the same signal, providing no geometrical resolution. Temporally scanning the beam between various fixed mirrors, to enable some dimensional resolution is described in U.S. Pat. No. 4,855,608, wherein a polygon scanning device is operated, enabling the scan of very slow moving objects, depending on the scanning velocity. U.S. Pat. No. 4,185,192 discloses a passive optical system utilizing two detectors oriented in such a way that their optical axes and cone shaped fields-of-view intersect, thereby creating an overlapping region between these fields-of-view. By affecting this, only those signals from the detectors, which are received simultaneously, are thereby indicative of the fact the detected light comes from the overlapping volume. Another example of a passive optical system for determining the presence of an object by utilizing the forming of a scene at the intersection of two optical paths associated with two detectors, is disclosed in U.S. Pat. No. 4,317,992. U.S. Pat. No. 4,396,945 discloses a technique that can be utilized in either an active or passive system for determining the position of an object in space. This technique is based on the determination of the intersection of a line with a plane or with another line. U.S. Pat. No. 4,590,410 discloses a system, which is aimed at detecting small objects. In this case, multiple light emitters and multiple light detectors are utilized. According to the technique described in U.S. Pat. Nos. 4,724,480, a system is composed of at least one projector generating a non-planar light and at least one camera, oriented such that the optical axes of the camera and projector intersect. In this way, the projector associated with a first object and the region of intersection associated with a second object, can be aligned. U.S. Pat. No. 6,943,337 discloses a system, where a laser forms a screen like plane, and camera-like detectors projecting substantially perpendicular to the plane of the screen, are faced at a selected area of the screen, to detect scattered light from an object passing through the screen. This geometry provides no information on the object velocity or orientation. None of the above systems enable the detection of the required parameters for detecting fast moving objects such as projectiles.
An optical screen for detection of position, time of passage and velocity of an object, e.g., a projectile, through the optical or light screen, has to have the following properties:                a) a capability of two and/or three-dimensional positioning;        b) a velocity determining capability to distinguish between slow and fast moving objections;        c) a multi-directional capability, or ability to measure the penetration angle;        d) not to be affected by sunlight or stray light, and lighted or shaded areas, and        e) able to detect objects above specified sized in diameter and/or length.        